The present invention is directed to an improved incipient fire detector employing a flow-through ionization particulate detector or sensor which is supplied with a controlled rate of flow of small particulates previously separated by aerodynamic particulate collector.
Fire detection systems and devices available today require the presence of flame, or the attainment of a preselected temperature level or the like, or rely instead on detection of fly ash or "smoke." A typical device which detects smoke and other combustion products uses an ionization particulate detector comprising two ionization chambers. A first, measuring chamber is exposed to the atmosphere where as a second, reference chamber is isolated from any fire-produced, atmospheric smoke or other combustion products. The reference chamber is employed to minimize the effects of normal ambient fluctuations in temperature, humidity and pressure on the operation of the measuring chamber.
Both ionization chambers employ a small radioactive source of ionization current. As the density of mass products of a fire increase in the atmosphere, they inhibit the travel of the ionized air molecules in the measuring chamber from the source to a spaced electrode. The resulting alteration in the ionization current of the measuring chamber in comparison to the current in the reference chamber can be used to indicate the existence of smoke and other combustion products.
In practice, variations in ionization current level due to smoke and other combustion products are extremely small. As a consequence, highly sensitive circuits are needed to reliably sense these changes. In an effort to enhance reliability, the measuring and reference ionization chambers of the prior art are often connected in series across a direct current voltage source. In addition, the reference chamber is reduced in size and redesigned to operate just below its physical and electrical saturation point. As a consequence, while small ambient changes in the atmospheric temperature, pressure or humidity have a nearly equal effect on both chambers, increased resistance in the measuring chamber due to the presence of fire-produced particles has an amplified effect on the total voltage distribution across both chambers which effect can more readily be detected as indicative of an existing fire.
Nevertheless, even these more advanced prior art ionization particulate detectors have inherent limitations which seriously affect their reliability and commercial utility. For one, the use of different sized or unbalanced ionization chambers results in saturation of the limited range reference chamber when the two chambers are exposed to extreme atmospheric temperature, pressure or humidity changes as can be expected when the detectors are called upon for service in aircraft, spacecraft or the like. In addition, such prior art detectors are adversely affected by uncontrolled variations in air current which can increase the density of particles within the measuring chamber or carry away the radiation field before it can ionize the air molecules which in either case results in a decrease in ionization current and accordingly a false alarm. Such uncontrolled variations in air current are caused, for instance, by the operation of an air conditioner or movement near the detector. As a consequence, diffusion shields are regularly positioned to protect the measuring ionization chambers from the effects of uncontrolled air current variations. These diffusion shields allow for entry of atmospheric particles to the measuring chamber only by the process of convection. Since the reaction time of the detector is dependant in part on the rate of air turn-over in the measuring chamber, the usefulness of detectors using diffusion shields is reduced proportionate to the inherent limitations of the convection process to enable smoke and other combustion products to enter and leave the measuring ionization chamber.
Even with a diffusion shield, the sensitivity of the measuring ionization chamber is further adversely affected by the presence of dust in the ambient atmosphere. Dust particles gather on both the radiation source and electrode of the measuring chamber resulting in a marked premature decrease in ionization current. Furthermore, dust suspended in the chamber itself acts to inhibit ion flow which results in a decrease of ionization current.
Screens and filters provide only a limited relief and introduce the added economic burden of keeping the screens and filters clean. Redesign of the radiation source reported in U.S. Pat. No. 3,353,170 provides only a partial solution.
A recently-developed fire detector system, instead of sensing smoke or other combustion products, keys on detecting micron-sized particulates known to be released into the air when combustible materials approach a state of fire, but before a state of fire actually exists. Such a device is disclosed in U.S. patent application Ser. No. 350,091, U.S. Pat. No. 3,953,844, filed on Apr. 11, 1973, by the present inventor as co-inventor and assigned to the assignee of the present invention. According to the general teachings of that application, a particulate collector is employed to allow only selectively small particulates of the variety typically generated in an incipient fire condition to impinge a crystal detector and thereby provide detection of those particulates to indicate the existence of the incipient fire condition.
The present invention improves upon the incipient detector of patent application Ser. No. 350,091 and effectively combines the aerodynamic particular collector disclosed therein with the innovative use of an ionization particle detector to result in a highly reliable, sensitive, commercially acceptable incipient fire detector.